In an internal-combustion engine vehicle, an emission gas purification catalyst is disposed to an exhaust pipe and an emission gas sensor that detects an air-fuel ratio of an emission gas or whether the emission gas is rich or lean is disposed upstream of the catalyst or both upstream and downstream of the catalyst. An emission gas purification ratio of the catalyst is increased by applying feedback control to an air-fuel ratio on the basis of an output of the emission gas sensor. Herein, the emission gas sensor is equivalent to an air-fuel ratio sensor or an oxygen sensor.
In the emission gas sensor, such as an oxygen sensor, when an air-fuel ratio of an emission gas varies between rich and lean, a sensor output varies in response to a variance of an actual air-fuel ratio with a lag in some cases.
In view of the circumstances, the gas sensor has a sensor element in which a solid electrolyte layer is disposed between sensor electrodes as disclosed, for example, in Patent Literature 1 (JP S60-98141 A). The gas sensor is equivalent to an oxygen sensor and the sensor electrodes have a reference electrode and an oxygen electrode. Output characteristics of the gas sensor are changed by controlling a current flowing from the reference electrode to the oxygen electrode using a current supply portion.